The present invention relates to a magnetic field device for a system for the acceleration and/or storage of electrically charged particles, especially of electrons, the particle trajectory of which has curved sections, in which respectively curved dipole magnets are arranged which contain superconducting windings and a supplemental winding and by which a magnetic guidance field for the particle beam can be generated which has a weakly-focusing action due to corresponding field gradients. Such a device is known, for instance, from the publication "Superconducting Racetrack Electron Storage Ring and Coexistent Injector Microtron for Synchrotron Radiation" of the "Institute for Solid State Physics" of the University of Tokyo, Japan, September 1984, Ser. B, No. 21, pages 1 to 29.
With known smaller circular electron accelerators, also called "Microtrons," particle energies up to about 100 MeV can be obtained. These systems can be realized also as so called racetrack microtrons. The particle trajectories of this type of accelerator are composed of two semi-circles, each with an appropriate 180.degree. deflection magnet and of two straight track sections (see also "Nucl. Instr. and Meth.," vol. 177, 1980, pages 411 to 416, or vol. 204, pages 1 to 20).
If the desired final energy of the electrons is to be increased from about 100 MeV to 1 GeV, one suggestion is to increase the magnetic field while leaving the dimensions alone. Such magnetic fields can be produced in particular by superconducting magnets.
Also the electron storage ring system from the publication first cited above has in its curved sections dipole magnets with superconducting windings. It is generally assumed there that the guiding field for the particle beam generated in the vicinity of these magnets has a weakly focusing action due to appropriate field gradients. A measure of this type of focusing is the so-called field index n, which is generally defined as: ##EQU1## where r.sub.o is the radius of the particle trajectory, B.sub.zO the component of the magnetic induction perpendicular relative to the particle trajectory, and .differential.B/.differential.r is the field gradient (see, for instance, R. Kollath: "Particle Accelerators," Braunschweig, 1955, page 23). In case of weak focusing, the field index is between about 0.3 and 0.7 and particularly approximately 0.5.
Such a weak focusing in the curved trajectory sections is generally achieved in known storage ring systems by special shapes of the pole pieces of an iron yoke of the dipole magnet surrounding the particle trajectory as well as, optionally, by special supplemental windings. Also in the storage ring system from the publication first cited above, the superconducting dipole magnets have iron yokes. These yokes are pierced outwards in the equatorial plane of the particle track in order to provide an outlet for and thereby, the utilization of the synchrotron radiation which occurs in the curved sections of the particle track.
Apart from the fact that in the known storage ring system, the formation of an appropriate iron yoke is comparatively expensive, also the contribution of the iron yoke to the magnetic flux density is limited upwards due to the magnetic saturation of the material.